State of the art gel spun UHMWPE multifilament yarns and a process for producing thereof are known from EP 1,699,954. The disclosure thereof relates to UHMWPE yarns having tensile strengths of up to 5.6 GPa and modulus of up to 203 GPa and containing at least 5 filaments.
Although such multifilament yarns have achieved wide acceptance in various fields of industry, there is still a need for further improved yarns and also for an improved process for producing thereof.
It is therefore a first object of the invention to provide novel gel spun UHMWPE multifilament yarns having improved physical and mechanical properties.
It is a second object of the invention to provide a process for producing the novel yarns, wherein the occurrence of disturbances and/or irregularities is reduced. Disturbances are unwanted occurrences that lead to stopping the process as for example filament breakage. Irregularities are unwanted occurrences that require changing the parameters of the process, e.g. spinning and drawing speeds, spinning rate and the like, to prevent alterations in the properties of the final yarn.
Surprisingly, it was found that the first objective was achieved with a novel and improved gel spun UHMWPE multifilament yarn characterized in that said yarn comprises individual monofilaments having a coefficient of variation of their linear density, hereafter CVintra, of less than 30%, wherein the CVintra of a monofilament was determined from linear density values corresponding to a number of 20 representative lengths randomly extracted by cutting from said monofilament and using Formula 1
                              CV          INTRA                =                                                                              ∑                                      i                    =                    1                                    n                                ⁢                                                      (                                                                  x                        i                                            -                                              x                        _                                                              )                                    2                                                            n                -                1                                              ×                      1                          x              _                                ×          100                                    Formula        ⁢                                  ⁢        1            wherein xi is the linear density of any one of the representative lengths extracted from the monofilament under investigation and x is the averaged linear density over the n=20 measured linear densities of said n=20 representative lengths.
The advantage of the yarn of the invention is that it is more homogeneous, i.e. the monofilaments of said yarn show less differentiation from one another in their mechanical and physical properties. The yarn of the invention has also improved mechanical and physical properties. Moreover, it was surprisingly found that the yarn of the invention shows improved handling, especially at elevated speeds as for example in coating processes or in processes including yarn winding and/or high speed yarn transportation. Examples of such processes wherein the yarn of the invention is successfully used include weaving, braiding and processes for the production of ropes, cables and nets, in particular knotless nets. Therefore, the invention also relates to the use of the yarn of the invention in processes including yarn winding and/or high speed yarn transportation.
Yet a further advantage of the yarn of the invention is that products comprising said yarn show improved mechanical properties. For example, a rope comprising said yarn shows improved fatigue and/or lifetime when subjected to for example cyclic loads. Another example is that of a medical cable and more in particular that of a suture comprising the yarn of the invention, said medical cable or suture showing for instance improved knot strength.
By mechanical properties of a yarn is herein understood those properties that are associated with an elastic or inelastic reaction of said yarn when a force is applied thereon. Examples of mechanical properties construed in the light of the present invention are tensile strength, elastic modulus, breaking force, elongation at break and the like. By physical properties is herein understood those properties characteristic to a yarn that can be observed or measured without changing the composition or identity of the yarn. Examples of physical properties construed in the light of the present invention are the linear density or the diameter of individual monofilaments, the titre of the yarn and the like.
For the purposes of the present invention, an individual monofilament is an elongated body the length dimension of which is much greater than its transverse diameter. Preferably, the monofilaments have a substantially circular or elliptical cross-section. By multifilament yarn is herein understood an elongated body comprising a plurality of individual monofilaments. The yarn of the invention may contain substantially parallel monofilaments or it may be twisted or braided.
Preferably, the CVintra of the inventive yarn is less than 25%, more preferably less than 20%, even more preferably less than 15%, yet even more preferably less than 10%, most preferably less than 5%. Multifilament UHMWPE yarns with such reduced CVintra values are for example obtained with the process of the invention as explained below.
Surprisingly, the forenamed advantages of the invention were also achieved according to a second embodiment of the invention with a novel gel spun UHMWPE multifilament yarn having a coefficient of variation in linear density between the monofilaments comprising said yarn, hereafter CVinter, of less than 50%, wherein CVinter is determined by using linear density values of a number of 50 representative lengths, wherein each of said lengths corresponds to a different randomly chosen monofilament and is extracted by cutting thereof and using Formula 2
                              CV          INTER                =                                                                              ∑                                      i                    =                    1                                    n                                ⁢                                                      (                                                                  x                        i                                            -                                              x                        _                                                              )                                    2                                                            n                -                1                                              ×                      1                          x              _                                ×          100                                    Formula        ⁢                                  ⁢        2            wherein xi is the linear density of any one of said representative lengths and x is the averaged linear density over the n=50 measured linear densities of the n=50 representative lengths corresponding to the randomly chosen monofilaments.
A surprising advantage of such a yarn is that for a determined tensile strength, said yarn has a reduced thickness compared with known yarns of the same strength. Without being bound to any explanation, the inventors attributed the reduction in thickness to a better packing of the individual monofilaments within the yarn.
Preferably, CVinter is less than 40%, more preferably less than 30%, even more preferably less than 20%, yet even more preferably less than 10%, most preferably less than 5%. Multifilament UHMWPE yarns with such reduced CVinter values are obtained for example with the process of the invention as explained below.
In a preferred embodiment of the invention the inventive yarns have both a CVintra and CVinter within the ranges defined above. Such yarns have further improved mechanical and/or physical properties.
Preferably, the modulus of the inventive yarns is at least 50 GPa, more preferably at least 100 GPa, even more preferably at least 150 GPa, most preferably at least 180 GPa.
Preferably, the strength of the inventive yarns is at least 1.2 GPa, more preferably at least 2 GPa, even more preferably at least 3 GPa, yet even more preferably at least 4 GPa, yet even more preferably at least 5 GPa, most preferably at least 5.5 GPa. The inventors were surprised that the inventive yarns have such high tensile strengths as it is known in the art that an increase in tensile properties is achieved at the expense of other physical properties, e.g. variations in their linear density. It was therefore, surprisingly found that the inventive yarns posses a combination high tensile strength and low CVinter and/or CVintra never achieved hitherto.
Preferably, the elongation at break of the inventive yarns is at most 5%, more preferably at most 3.5%, most preferably at most 2.5% and preferably at least 0.5%, more preferably at least 0.75%.
Preferably the titer of the individual monofilaments of the inventive yarns is at least 0.8 dpf, more preferably at least 1, most preferably at least 1.5 dpf. Preferably, said titer is at most 30 dpf, more preferably at most 20 dpf, most preferably at most 10 dpf. It is known in the art that the nonhomogeneity problems increase with decreasing the titer of the monofilaments. However, it was surprisingly found that the homogeneity of individual monofilaments expressed in terms of CVintra and the homogeneity of the yarn expressed in terms of CVinter are substantially preserved with decreasing the titer thereof.
By inventive yarns is hereinbefore and hereinafter understood the gel spun UHMWPE yarns of the invention. By representative lengths is understood lengths of monofilament randomly extracted by cutting either from the same monofilament under investigation when CVintra is to be determined, either each being extracted from a different monofilament of the yarn when CVinter is to be determined.
The invention also relates to articles comprising the novel and inventive gel spun UHMWPE multifilament yarns of the invention. It was found that ropes and nets comprising the yarns of the invention show improved properties and are easier to be manufactured from the yarns of the invention. Therefore, the invention relates in particular to a rope and a net comprising the inventive yarns. Ropes may be heavy-duty ropes, including ropes for application in marine and offshore operations, like anchor handling, seismic operations, mooring of drilling rigs and production platforms, and towing. The high tenacity and the high resistance of the yarn to wear give the rope an excellent load bearing performance. The rope is easy to handle because of its light-weight. The net may be a fishing net. High bite-resistance and light-weight of the yarn makes it especially useful as a fishing net.
The invention also relates to medical devices comprising the yarns of the invention. In a preferred embodiment, the medical device is a cable or a suture. Other examples include mesh, endless loop products, bag-like, balloon-like products and other woven and/or knitted products. Good examples of cables include a trauma fixation cable, a sternum closure cable, and a prophylactic or per prosthetic cable, long bone fracture fixation cable, small bone fracture fixation cable. Also tube-like products for e.g. ligament replacement are possible.
Composite articles comprising the yarns of the invention also show improved properties. Therefore, the invention relates in particular to a composite article comprising the yarns in accordance with the embodiments of the invention. Preferably, the composite articles comprise networks of the inventive yarns. By network is meant that the monofilaments of said yarns are arranged in configurations of various types, e.g. a knitted or woven fabric, a non-woven fabric with a random or ordered orientation of the yarns, a parallel array arrangement also known as unidirectional UD arrangement, layered or formed into a fabric by any of a variety of conventional techniques. Preferably, said articles comprise at least one network of said yarns. More preferably, said articles comprise a plurality of networks of the inventive yarns, preferably UD networks and preferably the direction of the yarns in one layer being at an angle to the direction of the yarns in adjacent layers. Such networks of the inventive yarns can be comprised in cut resistant garments, e.g. gloves and also in anti-ballistic products, e.g. bullet-proof vests and helmets. Therefore, the invention also relates to the articles enumerated hereinabove comprising the yarns of the invention.
The invention also relates to a roundsling comprising the yarn of the invention. Since roundslings need to be able to withstand forces in severe conditions, often for a long time, the high strength of the yarn is advantageous.
The invention also relates to sports equipments comprising the yarn of the invention, including a fishing line, a kite line and a yacht line. The low elongation and high modulus of the yarn are advantageous for a fishing line, because it allows a fisherman to feel even an initial bite of a fish on a lure. These properties also allow precise control in kiting and yachting.
The invention also relates to an air cargo net and an air freight container comprising the yarn of the invention. The high strength, abrasion resistance and lightweight of the yarn make it especially suitable in an aircraft application.
The invention further relates to a gel spinning process of producing the novel and inventive UHMWPE multifilament yarns. The process according to the invention comprises the steps of:    a) feeding to an extruder a slurry containing an UHMWPE in a spinning solvent;    b) converting the slurry in the extruder into a solution of UHMWPE in the spinning solvent;    c) spinning a multifilament yarn by passing the solution of step b) through a spinning plate containing a plurality of spin holes to form the monofilaments comprising said yarn;    d) cooling the obtained monofilaments to form gel monofilaments;    e) removing at least partly the spinning solvent from the gel monofilaments; and    f) drawing the monofilaments in at least one drawing step before, during or after removing the spin solvent;characterised in that a chamber is present before the spinning plate such that no further partitioning of the UHMWPE solution obtained at step b) takes place before said solution being finally partitioned into individual monofilaments in step c) and in which chamber the solution has a residence time τ at a constant throughput of UHMWPE solution of at least 50 sec.
By partitioning of the UHMWPE solution is herein meant dividing the volume of said solution into a plurality of smaller volumes for example by the teeth of moving components in an extruder, gear pump, positive displacement pump and the like or by passing the solution through a filtering sieve, through multiple conduits at the same time, and the like.
By residence time τ is herein understood the average time (in seconds) spent by a volume unit of the UHMWPE solution within the chamber before exiting it. The residence time is defined as the ratio between the volume V of the chamber and the volumetric flow rate ν according to Formula 3:
                    τ        =                  V          v                                    Formula        ⁢                                  ⁢        3            
The volumetric flow rate ν is the volume of UHMWPE solution exiting the nozzle of the extruder, i.e. the output of the extruder, perpendicularly flowing through the cross-section of the chamber per unit time.
It has been surprisingly found that the process of the invention produces new and improved UHMWPE multifilament yarns and is less adversely affected by disturbances and/or irregularities as compared with known processes. It was found that disturbances and/or irregularities were present in the production process to a lesser extent making the process more economical. It was also found that the number of events wherein the total breakage of the yarns occurred was reduced also. Surprisingly, the yarns of the invention were produced with an improved throughput than the known gel spun UHMWPE multifilament yarns.
Also an improved yield at the same production velocity was surprisingly observed. The process of the invention thus produces a yarn characterized by a low CVinter and/or CVintra even when using a large number of spin holes and furthermore it operates much more economically than other, comparable processes.
A process comprising the steps a)-f) is known from EP 1,699,954. However, the disclosure thereof does not mention a chamber wherein the UHMWPE solution resides for a time τ.
WO 2007/118008 A2 discloses the use of a chamber to introduce a residence time in a gel spinning process for UHMWPE. However, the process disclosed therein uses the residence time to allow for a longer dissolution time of the particles of the UHMWPE powder in the spinning solvent. Said process does not use the residence time to allow for a longer relaxation time of the UHMWPE solution obtained after the dissolution of said particles in said solvent and/or after the extrusion step, as the process of the invention allows by using a chamber as specified in the paragraphs above. Furthermore, in the process of the cited reference, after making the UHMWPE solution, said solution is passed through a positive displacement pump wherein partitioning of the solution takes place. Therefore, the advantageous effects of the process of the invention cannot be achieved by the process disclosed in the cited reference. Hereafter the figures are explained.